stringobject.c's formatfloat seems confused in a
couple respects.
1. Testing "fabs(x)/1e25 >= 1e25" is pretty baffling.
What is it intending to do? If it means what it says,
it should be the simpler "fabs(x) >= 1e50". But maybe
it really intended to test "fabs(x) >= 1e25".
2. The "worst case length calc" is fantasy in some %f
cases. It assumes there's one digit before the
decimal point, but, e.g., '%.100f'% 1e49 produces
dozens of digits before the decimal point. We're
saved from a buffer overrun thanks to the PyOS_snprintf
() following, but unclear that truncation is sensible
here (e.g., the user asked for a precision of 100
here, but only gets back 50 digits after the decimal
point).
Complication: we're deliberately replacing C's %f
with C's %g in some cases, but the docs don't document
the rules Python intends for %f.